1. Field of the Invention
The present invention relates to a technique which enables an operation amount or a circuit scale to be reduced when processing of enlarging or reducing a digital image is carried out.
2. Description of the Related Art
Recently, digital type image pickup devices, for example, a video camera, and a still picture video camera have rapidly propagated. Accordingly, a signal processing technique containing a video signal is also shifted from analog to digital, and images to be processed have been changed from analog images recorded on silver chloride films to digital images recorded in various types of digital recording media.
In a case where analog images and digital images as described above are enlarged or reduced, the processing based on an analog means using an optical system or the like is carried out in the former case. However, in the latter case, it is necessary to determine image constituent pixel values through operations to carry out the digital processing.
That is, as shown in FIG. 3, an interpolation continuous signal obtained by arranging interpolation signals restored through an interpolation operation between discrete signals constituting an original image is enlarged by re-sampling the image data at smaller intervals than the original image when the image is enlarged, and also reduced by re-sampling the image data at larger intervals than the original image when the image is reduced. In the above interpolating operation, the interpolation using the sinc function based on the sampling theorem is the basic concept.
In the conventional interpolating operation is proposed an interpolating method of using an approximate expression of the sinc function as an interpolating function for convolution in order to reduce the operation amount. As such an interpolating method are known a nearest neighbor method, a bilinear method, a cubic convolution method, etc.
The operation based on the convolution is implemented by a so-called FIR digital filter. The image quality of an enlarged or reduced image obtained by the nearest neighbor method, the bilinear method or the cubic convolution method is enhanced more and more in this order, however, the order of the interpolating function and the number of taps of a filter are increased more and more in this order.
Accordingly, when the operation based on each interpolating operation method is implemented by software, the operation amount is increased in the above order, that is, in the order of enhancing the image quality, so that there is a problem that the load imposed on an operating unit such as CPU is increased and thus the processing time is long. Further, when the operation is carried out by hardware, there is a problem that the circuit scale is likewise increased.